Neurosurgeons are required to perform meticulous drilling procedures in various regions of the skull without disturbing critical structures such as nerves, vessels, and most importantly the brain. When surgeons drill through the skull they carefully shave down the bone, layer by layer until they reach the dura, the thin protective covering of the brain compartment. This can be very time consuming as surgeons spend up to half an hour drilling through the final portions of bone because of the grave implications of drilling through the dura. One clue that neurosurgeons frequently use during a drilling procedure to ascertain the proximity of the drill to the dura are the drill sounds. The acoustical pitches generated by the vibrating bone have been observed to change as different bone layers are penetrated. In fact, many surgeons teach their trainees to listen specifically for these cues. Although the most experienced neurosurgeons are able to utilize these acoustical cues, the reality of human error compels these surgeons to drill slowly once a certain depth in the bone is reached.
Improvements in identifying the location of a drill relative to target areas of the patient's body are desirable to improve efficiency and accuracy of surgical procedures. In addition, improvements in identifying the physical state and condition of surgical tools are also desirable to ensure proper operation of those tools.